I've seen it before but it was never consistently reproducible that I could draw some conclusion. After seeing your post, I just did 2 rounds of Test Create Volume on cm_storage command and I got it in 2nd time, first one was lightening quick in comparison. Is it consistent for you Francois? What is your browser? Are you running your server on non-default ports?
06:52:54.860 INFO [Create volume] ### Command 'Create volume' ###
Have you tried running Powershell itself and time it, then load the "profile.ps1" in the <WFA>/PoSH folder and time that. (cd <wfa>\PoSH ;; .\profile.ps1)
I've found installing additional powershell extensions on the WFA machine, when there were configuration issues, or even the built-in ones, can cause long delays to launching and some logging commands.
Each time a command is run, even a No-Op, a new powershell.exe instance is launched and loads the profile that loads additional modules. If you run a lot of commands and there's a delay there, that time can add up.